Some properties of the reflectivity and transmissivity of multilayer dielec
tric optical coatings are derived from energy conservation applied to a bea
m splitter acting as a beam combiner. It is shown that dispersion in the ph
ase shifts of the beam splitter coatings influences which quadrature amplit
ude is measured in a homodyne detector, and, in a Michelson interferometer,
how the white-light fringe pattern is disposed about the zero-path-differe
nce condition. The functions of reflectivity and transmissivity that determ
ine these properties are shown to be not completely constrained by consider
ations of energy conservation and time reversibility, in contrast to common
ly employed models for beam splitter reflectivity and transmissivity. Some
representative beam splitter coating designs are used to illustrate this po
int. (C) 2000 American Association of Physics Teachers.